1. Field of the Invention
This invention relates to an active replication transmitter circuit for near end transmission cancellation and more specifically to an adjustable replication transmitter circuit with a low pass filter.
2. Background Information
Hybrids are widely used in communication systems that send and receive signals on a single pair of wires. In order to detect the signals with error free performance, it is important that near end transmission from the nearby-transmitted signal be removed from the received signal. A good hybrid is defined as a hybrid that can reject most of the near end transmission signal from the desired received signal. A good hybrid is important, because as the distance between two ends of a communicating/network nodes increases, the received signal strength decreases, while near end transmission signal from the nearby transmitted signal stays approximately constant. In order to detect the received signal with error free performance, it is important that any near end transmission signal from the nearby transmitted signal is removed from the received signal.
In many conventional communications systems, such as PC modems, ADSL, VDSL and the like, operating on standard twisted pair telephone wires, the two ends of the communicating nodes are isolated by one or more isolation transformers. There are two types of conventional hybrids typically used in such applications. The first is a bridge hybrid, also referred to as a resistive bridge hybrid. The second type is a hybrid transformer. For these types of applications, these hybrids generally work well, because these applications do not usually utilize the low frequency range of the communications bandwidth. Another term for this is DC free signaling.
Newer communication systems, such as gigabit Ethernet (I.E.E.E. standard 802.3ab), use a non-DC free signaling. Unfortunately, conventional hybrids only work well for rejecting higher frequency near end transmission signals. Therefore, extremely complicated digital signal processing (DSP) based echo canceling technology is needed to reject not only the residual high frequency echo but also the large amplitude low frequency echo signal. This low frequency echo signal is seen by the receiver as transmitter base line wander.
U.S. Pat. No. 4,935,919 to Hiraguchi is directed to an echo canceler in a modem, which cancels echoes from hybrid transformers on both the near end and the far end. The echo canceler has a variable delay, which may be adjusted to conform to a round trip of an echo. An adaptive filter has a number of delay circuits, each adding an increment of delay. A number of these delay circuits are selected in order to provide a selected delay time.
U.S. Pat. No. 5,305,379 to Takeuchi, et al. describes a sending data buffer for holding sending data temporarily and transmitting the data to an echo canceler section. The data buffer is installed between a sending section and an echo canceler that is included in a subscriber line circuit of an integrated service digital network. The sending data buffer is operated in a shift register mode during a sending training mode, and operated in FIFO (first in-first out) mode during a sending/receiving training mode.
FIG. 1 shows a block diagram of a communication system showing a near end transmitter 12(NET), near end receiver 14 (NER), in communication with a near end hybrid 10. A wire link 16, usually a twisted wire pair, connects the near end hybrid to a far end hybrid 11 which is in communication with a far end receiver 15 (FER) and a far end transmitter 13 (FET). Desired transmission is from the near end transmitter 12 to the far end receiver 15 and from the far end transmitter 13 to the near end receiver 14. It is important to reject or attenuate near end transmission signals from the near end transmitter 12 to the near end receiver 14.
FIG. 2A shows a diagram of the near end of such a communication system having cross talk attenuation. In this arrangement, NET 12 is configured as a current source. Current generated by the current source flows through output resistor R and develops a voltage across output resistor R. Alternatively, as shown in FIG. 2B, NET 12′ may be configured as a voltage source having resistors R1 and R2. In either arrangement, NET 12 (12′) feeds the primary of an isolation transformer 20. The secondary of the isolation transformer 20 is connected to a twisted wire pair communication link 22, which will be connected to a far end circuit, not shown. A replication transmitter 18 is provided to attempt to eliminate near end transmission signals from transmitter 12 (12′). The output of replication transmitter 18 is subtracted from the primary of the isolation transformer 20 by subtraction circuit 24. The output of subtraction circuit 24 is provided as an input to NER 14. Thus, the input NER 14 comprises the received signal and the transmitted signal less the replication signal. In order to eliminate effectively the effects of NET 12, the voltage developed at the output of NET 12 (I×R) should be equal to the voltage developed by replication transmitter 18 or I×Rreplication. In other words, Rreplication should be equal to R. However, due to process variations, it is difficult to ensure that Rreplication is equal to R. As a result, such a conventional arrangement does not sufficiently eliminate the effects from NET 12 (12′).